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RFC 8264

PRECIS Framework: Preparation, Enforcement, and Comparison of Internationalized Strings in Application Protocols

Pages: 43
Proposed STD
Obsoletes:  7564
Part 2 of 3 – Pages 14 to 29
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Top   ToC   RFC8264 - Page 14   prevText
5.  Profiles

   This framework document defines the valid, contextual rule required,
   disallowed, and unassigned rules for the IdentifierClass and the
   FreeformClass.  A profile of a PRECIS string class MUST define the
   width mapping, additional mapping (if any), case mapping,
   normalization, and directionality rules.  A profile MAY also restrict
   the allowable code points above and beyond the definition of the
   relevant PRECIS string class (but MUST NOT add as valid any code
   points that are disallowed by the relevant PRECIS string class).
   These matters are discussed in the following subsections.

   Profiles of the PRECIS string classes are registered with the IANA as
   described under Section 11.3.  Profile names use the following
   convention: they are of the form "Profilename of BaseClass", where
   the "Profilename" string is a differentiator and "BaseClass" is the
   name of the PRECIS string class being profiled; for example, the
   profile used for opaque strings such as passwords is the OpaqueString
   profile of the FreeformClass [RFC8265].

5.1.  Profiles Must Not Be Multiplied beyond Necessity

   The risk of profile proliferation is significant because having too
   many profiles will result in different behavior across various
   applications, thus violating what is known in user interface design
   as the "Principle of Least Astonishment".

   Indeed, we already have too many profiles.  Ideally, we would have at
   most two or three profiles.  Unfortunately, numerous application
   protocols exist with their own quirks regarding protocol strings.
   Domain names, email addresses, instant messaging addresses, chat room
   names, user nicknames or display names, filenames, authentication
   identifiers, passwords, and other strings already exist in the wild
   and need to be supported in existing application protocols such as
   DNS, SMTP, the Extensible Messaging and Presence Protocol (XMPP),
   Internet Relay Chat (IRC), NFS, the Internet Small Computer System
   Interface (iSCSI), the Extensible Authentication Protocol (EAP), and
   the Simple Authentication and Security Layer (SASL) [RFC4422], among

   Nevertheless, profiles must not be multiplied beyond necessity.

   To help prevent profile proliferation, this document recommends
   sensible defaults for the various options offered to profile creators
   (such as width mapping and Unicode normalization).  In addition, the
   guidelines for designated experts provided under Section 10 are meant
   to encourage a high level of due diligence regarding new profiles.
Top   ToC   RFC8264 - Page 15
5.2.  Rules

5.2.1.  Width Mapping Rule

   The width mapping rule of a profile specifies whether width mapping
   is performed on a string and how the mapping is done.  Typically,
   such mapping consists of mapping fullwidth and halfwidth code points,
   i.e., code points with a Decomposition Type of Wide or Narrow, to
   their decomposition mappings; as an example, "0" (FULLWIDTH DIGIT
   ZERO, U+FF10) would be mapped to "0" (DIGIT ZERO U+0030).

   The normalization form specified by a profile (see below) has an
   impact on the need for width mapping.  Because width mapping is
   performed as a part of compatibility decomposition, a profile
   employing either Normalization Form KD (NFKD) or Normalization
   Form KC (NFKC) does not need to specify width mapping.  However, if
   Unicode Normalization Form C (NFC) is used (as is recommended), then
   the profile needs to specify whether to apply width mapping; in this
   case, width mapping is in general RECOMMENDED because allowing
   fullwidth and halfwidth code points to remain unmapped to their
   compatibility variants would violate the "Principle of Least
   Astonishment".  For more information about the concept of width in
   East Asian scripts within Unicode, see Unicode Standard Annex #11

      Note: Because the East Asian width property is not guaranteed to
      be stable by the Unicode Standard (see
      <> for details),
      the results of applying a given width mapping rule might not be
      consistent across different versions of Unicode.

5.2.2.  Additional Mapping Rule

   The additional mapping rule of a profile specifies whether additional
   mappings are performed on a string, such as:

   o  Mapping of delimiter code points (such as '@', ':', '/', '+',
      and '-').

   o  Mapping of special code points (e.g., non-ASCII space code points
      to SPACE (U+0020) or control code points to nothing).

   The PRECIS mappings document [RFC7790] describes such mappings in
   more detail.
Top   ToC   RFC8264 - Page 16
5.2.3.  Case Mapping Rule

   The case mapping rule of a profile specifies whether case mapping
   (instead of case preservation) is performed on a string and how the
   mapping is applied (e.g., mapping uppercase and titlecase code points
   to their lowercase equivalents).

   If case mapping is desired (instead of case preservation), it is
   RECOMMENDED to use the Unicode toLowerCase() operation defined in the
   Unicode Standard [Unicode].  In contrast to the Unicode toCaseFold()
   operation, the toLowerCase() operation is less likely to violate the
   "Principle of Least Astonishment", especially when an application
   merely wishes to convert uppercase and titlecase code points to their
   lowercase equivalents while preserving lowercase code points.
   Although the toCaseFold() operation can be appropriate when an
   application needs to compare two strings (such as in search
   operations), in general few application developers and even fewer
   users understand its implications, so toLowerCase() is almost always
   the safer choice.

      Note: Neither toLowerCase() nor toCaseFold() is designed to handle
      various language-specific issues, such as the character "ı" (LATIN
      SMALL LETTER DOTLESS I, U+0131) in several Turkic languages.  The
      reader is referred to the PRECIS mappings document [RFC7790],
      which describes these issues in greater detail.

   In order to maximize entropy and minimize the potential for false
   accepts, it is NOT RECOMMENDED for application protocols to map
   uppercase and titlecase code points to their lowercase equivalents
   when strings conforming to the FreeformClass, or a profile thereof,
   are used in passwords; instead, it is RECOMMENDED to preserve the
   case of all code points contained in such strings and then perform
   case-sensitive comparison.  See also the related discussion in
   Section 12.6 of this document and in [RFC8265].

5.2.4.  Normalization Rule

   The normalization rule of a profile specifies which Unicode
   Normalization Form (D, KD, C, or KC) is to be applied (see Unicode
   Standard Annex #15 [UAX15] for background information).

   In accordance with [RFC5198], Normalization Form C (NFC) is

   Protocol designers and application developers need to understand that
   certain Unicode normalization forms, especially NFKC and NFKD, can
   result in significant loss of information in various circumstances
   and that these circumstances can depend on the language and script of
Top   ToC   RFC8264 - Page 17
   the strings to which the normalization forms are applied.  Extreme
   care should be taken when specifying the use of these normalization

5.2.5.  Directionality Rule

   The directionality rule of a profile specifies how to treat strings
   containing what are often called "right-to-left" (RTL) code points
   (see Unicode Standard Annex #9 [UAX9]).  RTL code points come from
   scripts that are normally written from right to left and are
   considered by Unicode to, themselves, have right-to-left
   directionality.  Some strings containing RTL code points also contain
   "left-to-right" (LTR) code points, such as ASCII numerals, as well as
   code points without directional properties.  Consequently, such
   strings are known as "bidirectional strings".

   Presenting bidirectional strings in different layout systems (e.g., a
   user interface that is configured to handle primarily an RTL script
   vs. an interface that is configured to handle primarily an LTR
   script) can yield display results that, while predictable to those
   who understand the display rules, are counterintuitive to casual
   users.  In particular, the same bidirectional string (in PRECIS
   terms) might not be presented in the same way to users of those
   different layout systems, even though the presentation is consistent
   within any particular layout system.  In some applications, these
   presentation differences might be considered problematic and thus the
   application designers might wish to restrict the use of bidirectional
   strings by specifying a directionality rule.  In other applications,
   these presentation differences might not be considered problematic
   (this especially tends to be true of more "free-form" strings) and
   thus no directionality rule is needed.

   The PRECIS framework does not directly address how to deal with
   bidirectional strings across all string classes and profiles nor does
   it define any new directionality rules, because at present there is
   no widely accepted and implemented solution for the safe display of
   arbitrary bidirectional strings beyond the Unicode bidirectional
   algorithm [UAX9].  Although rules for management and display of
   bidirectional strings have been defined for domain name labels and
   similar identifiers through the "Bidi Rule" specified in the IDNA2008
   specification on right-to-left scripts [RFC5893], those rules are
   quite restrictive and are not necessarily applicable to all
   bidirectional strings.

   The authors of a PRECIS profile might believe that they need to
   define a new directionality rule of their own.  Because of the
   complexity of the issues involved, such a belief is almost always
   misguided, even if the authors have done a great deal of careful
Top   ToC   RFC8264 - Page 18
   research into the challenges of displaying bidirectional strings.
   This document strongly suggests that profile authors who are thinking
   about defining a new directionality rule should think again and
   instead consider using the "Bidi Rule" [RFC5893] (for profiles based
   on the IdentifierClass) or following the Unicode bidirectional
   algorithm [UAX9] (for profiles based on the FreeformClass or in
   situations where the IdentifierClass is not appropriate).

5.3.  A Note about Spaces

   With regard to the IdentifierClass, the consensus of the PRECIS
   Working Group was that spaces are problematic for many reasons,
   including the following:

   o  Many Unicode code points are confusable with SPACE (U+0020).

   o  Even if non-ASCII space code points are mapped to SPACE (U+0020),
      space code points are often not rendered in user interfaces,
      leading to the possibility that a human user might consider a
      string containing spaces to be equivalent to the same string
      without spaces.

   o  In some locales, some devices are known to generate a code point
      other than SPACE (U+0020), such as ZERO WIDTH JOINER (U+200D),
      when a user performs an action like pressing the space bar on a

   One consequence of disallowing space code points in the
   IdentifierClass might be to effectively discourage their use within
   identifiers created in newer application protocols; given the
   challenges involved with properly handling space code points
   (especially non-ASCII space code points) in identifiers and other
   protocol strings, the PRECIS Working Group considered this to be a
   feature, not a bug.

   However, the FreeformClass does allow spaces; this in turn enables
   application protocols to define profiles of the FreeformClass that
   are more flexible than any profiles of the IdentifierClass.  In
   addition, as explained in Section 6.3, application protocols can also
   define application-layer constructs containing spaces.

6.  Applications

6.1.  How to Use PRECIS in Applications

   Although PRECIS has been designed with applications in mind,
   internationalization is not suddenly made easy through the use of
   PRECIS.  Indeed, because it is extremely difficult for protocol
Top   ToC   RFC8264 - Page 19
   designers and application developers to do the right thing for all
   users when supporting internationalized strings, often the safest
   option is to support only the ASCII range [RFC20] in various protocol
   slots.  This state of affairs is unfortunate but is the direct result
   of the complexities involved with human languages (e.g., the vast
   number of code points, scripts, user communities, and rules with
   their inevitable exceptions), which kinds of strings application
   developers and their users wish to support, the wide range of devices
   that users employ to access services enabled by various Internet
   protocols, and so on.

   Despite these significant challenges, application and protocol
   developers sometimes persevere in attempting to support
   internationalized strings in their systems.  These developers need to
   think carefully about how they will use the PRECIS string classes, or
   profiles thereof, in their applications.  This section provides some
   guidelines to application developers (and to expert reviewers of
   application-protocol specifications).

   o  Don't define your own profile unless absolutely necessary (see
      Section 5.1).  Existing profiles have been designed for wide
      reuse.  It is highly likely that an existing profile will meet
      your needs, especially given the ability to specify further
      excluded code points (Section 6.2) and to build application-layer
      constructs (see Section 6.3).

   o  Do specify:

      *  Exactly which entities are responsible for preparation,
         enforcement, and comparison of internationalized strings (e.g.,
         servers or clients).

      *  Exactly when those entities need to complete their tasks (e.g.,
         a server might need to enforce the rules of a profile before
         allowing a client to gain network access).

      *  Exactly which protocol slots need to be checked against which
         profiles (e.g., checking the address of a message's intended
         recipient against the UsernameCaseMapped profile [RFC8265] of
         the IdentifierClass or checking the password of a user against
         the OpaqueString profile [RFC8265] of the FreeformClass).

      See [RFC8265] and [RFC7622] for definitions of these matters for
      several applications.
Top   ToC   RFC8264 - Page 20
6.2.  Further Excluded Characters

   An application protocol that uses a profile MAY specify particular
   code points that are not allowed in relevant slots within that
   application protocol, above and beyond those excluded by the string
   class or profile.

   That is, an application protocol MAY do either of the following:

   1.  Exclude specific code points that are allowed by the relevant
       string class.

   2.  Exclude code points matching certain Unicode properties (e.g.,
       math symbols) that are included in the relevant PRECIS string

   As a result of such exclusions, code points that are defined as valid
   for the PRECIS string class or profile will be defined as disallowed
   for the relevant protocol slot.

   Typically, such exclusions are defined for the purpose of backward
   compatibility with legacy formats within an application protocol.
   These are defined for application protocols, not profiles, in order
   to prevent multiplication of profiles beyond necessity (see
   Section 5.1).

6.3.  Building Application-Layer Constructs

   Sometimes, an application-layer construct does not map in a
   straightforward manner to one of the PRECIS string classes or a
   profile thereof.  Consider, for example, the "simple username"
   construct in SASL [RFC4422].  Depending on the deployment, a simple
   username might take the form of a user's full name (e.g., the user's
   personal name followed by a space and then the user's family name).
   Such a simple username cannot be defined as an instance of the
   IdentifierClass or a profile thereof, because space code points are
   not allowed in the IdentifierClass; however, it could be defined
   using a space-separated sequence of IdentifierClass instances, as in
   the following ABNF [RFC5234] from [RFC8265]:

      username   = userpart *(1*SP userpart)
      userpart   = 1*(idpoint)
                   ; an "idpoint" is a Unicode code point that
                   ; can be contained in a string conforming to
                   ; the PRECIS IdentifierClass
Top   ToC   RFC8264 - Page 21
   Similar techniques could be used to define many application-layer
   constructs, say of the form "user@domain" or "/path/to/file".

7.  Order of Operations

   To ensure proper comparison, the rules specified for a particular
   string class or profile MUST be applied in the following order:

   1.  Width Mapping Rule

   2.  Additional Mapping Rule

   3.  Case Mapping Rule

   4.  Normalization Rule

   5.  Directionality Rule

   6.  Behavioral rules for determining whether a code point is valid,
       allowed under a contextual rule, disallowed, or unassigned

   As already described, the width mapping, additional mapping, case
   mapping, normalization, and directionality rules are specified for
   each profile, whereas the behavioral rules are specified for each
   string class.  Some of the logic behind this order is provided under
   Section 5.2.1 (see also the PRECIS mappings document [RFC7790]).  In
   addition, this order is consistent with IDNA2008, and with both
   IDNA2003 and Stringprep before then, for the purpose of enabling code
   reuse and of ensuring as much continuity as possible with the
   Stringprep profiles that are obsoleted by several PRECIS profiles.

   Because of the order of operations specified here, applying the rules
   for any given PRECIS profile is not necessarily an idempotent
   procedure (e.g., under certain circumstances, such as when Unicode
   Normalization Form KC is used, performing Unicode normalization after
   case mapping can still yield uppercase characters for certain code
   points).  Therefore, an implementation SHOULD apply the rules
   repeatedly until the output string is stable; if the output string
   does not stabilize after reapplying the rules three (3) additional
   times after the first application, the implementation SHOULD
   terminate application of the rules and reject the input string as

8.  Code Point Properties

   In order to implement the string classes described above, this
   document does the following:
Top   ToC   RFC8264 - Page 22
   1.  Reviews and classifies the collections of code points in the
       Unicode coded character set by examining various code point

   2.  Defines an algorithm for determining a derived property value,
       which can depend on the string class being used by the relevant
       application protocol.

   This document is not intended to specify precisely how derived
   property values are to be applied in protocol strings.  That
   information is the responsibility of the protocol specification that
   uses or profiles a PRECIS string class from this document.  The value
   of the property is to be interpreted as follows.

   PROTOCOL VALID  Those code points that are allowed to be used in any
      PRECIS string class (currently, IdentifierClass and
      FreeformClass).  The abbreviated term "PVALID" is used to refer to
      this value in the remainder of this document.

   SPECIFIC CLASS PROTOCOL VALID  Those code points that are allowed to
      be used in specific string classes.  In the remainder of this
      document, the abbreviated term *_PVAL is used, where * = (ID |
      FREE), i.e., either "FREE_PVAL" for the FreeformClass or "ID_PVAL"
      for the IdentifierClass.  In practice, the derived property
      ID_PVAL is not used in this specification, because every ID_PVAL
      code point is PVALID.

   CONTEXTUAL RULE REQUIRED  Some characteristics of the code point,
      such as its being invisible in certain contexts or problematic in
      others, require that it not be used in a string unless specific
      other code points or properties are present in the string.  As in
      IDNA2008, there are two subdivisions of CONTEXTUAL RULE REQUIRED:
      the first for Join_controls (called "CONTEXTJ") and the second for
      other code points (called "CONTEXTO").  A string MUST NOT contain
      any characters whose validity is context-dependent, unless the
      validity is positively confirmed by a contextual rule.  To check
      this, each code point identified as CONTEXTJ or CONTEXTO in the
      "PRECIS Derived Property Value" registry (Section 11.1) MUST have
      a non-null rule.  If such a code point is missing a rule, the
      string is invalid.  If the rule exists but the result of applying
      the rule is negative or inconclusive, the proposed string is
      invalid.  The most notable of the CONTEXTUAL RULE REQUIRED code
      points are the Join Control code points ZERO WIDTH JOINER (U+200D)
      and ZERO WIDTH NON-JOINER (U+200C), which have a derived property
      value of CONTEXTJ.  See Appendix A of [RFC5892] for more
Top   ToC   RFC8264 - Page 23
   DISALLOWED  Those code points that are not permitted in any PRECIS
      string class.

   SPECIFIC CLASS DISALLOWED  Those code points that are not to be
      included in one of the string classes but that might be permitted
      in others.  In the remainder of this document, the abbreviated
      term *_DIS is used, where * = (ID | FREE), i.e., either "FREE_DIS"
      for the FreeformClass or "ID_DIS" for the IdentifierClass.  In
      practice, the derived property FREE_DIS is not used in this
      specification, because every FREE_DIS code point is DISALLOWED.

   UNASSIGNED  Those code points that are not designated (i.e., are
      unassigned) in the Unicode Standard.

   The algorithm to calculate the value of the derived property is as
   follows (implementations MUST NOT modify the order of operations
   within this algorithm, because doing so would cause inconsistent
   results across implementations):

   If .cp. .in. Exceptions Then Exceptions(cp);
   Else If .cp. .in. BackwardCompatible Then BackwardCompatible(cp);
   Else If .cp. .in. Unassigned Then UNASSIGNED;
   Else If .cp. .in. ASCII7 Then PVALID;
   Else If .cp. .in. JoinControl Then CONTEXTJ;
   Else If .cp. .in. OldHangulJamo Then DISALLOWED;
   Else If .cp. .in. PrecisIgnorableProperties Then DISALLOWED;
   Else If .cp. .in. Controls Then DISALLOWED;
   Else If .cp. .in. HasCompat Then ID_DIS or FREE_PVAL;
   Else If .cp. .in. LetterDigits Then PVALID;
   Else If .cp. .in. OtherLetterDigits Then ID_DIS or FREE_PVAL;
   Else If .cp. .in. Spaces Then ID_DIS or FREE_PVAL;
   Else If .cp. .in. Symbols Then ID_DIS or FREE_PVAL;
   Else If .cp. .in. Punctuation Then ID_DIS or FREE_PVAL;

   The value of the derived property calculated can depend on the string
   class; for example, if an identifier used in an application protocol
   is defined as profiling the PRECIS IdentifierClass then a space
   character such as SPACE (U+0020) would be assigned to ID_DIS, whereas
   if an identifier is defined as profiling the PRECIS FreeformClass
   then the character would be assigned to FREE_PVAL.  For the sake of
   brevity, the designation "FREE_PVAL" is used herein, instead of the
   longer designation "ID_DIS or FREE_PVAL".  In practice, the derived
   properties ID_PVAL and FREE_DIS are not used in this specification,
   because every ID_PVAL code point is PVALID and every FREE_DIS code
   point is DISALLOWED.
Top   ToC   RFC8264 - Page 24
   Use of the name of a rule (such as "Exceptions") implies the set of
   code points that the rule defines, whereas the same name as a
   function call (such as "Exceptions(cp)") implies the value that the
   code point has in the Exceptions table.

   The mechanisms described here allow determination of the value of the
   property for future versions of Unicode (including code points added
   after Unicode 5.2 or 7.0, depending on the category, because some
   categories mentioned in this document are simply pointers to IDNA2008
   and therefore were defined at the time of Unicode 5.2).  Changes in
   Unicode properties that do not affect the outcome of this process
   therefore do not affect this framework.  For example, a code point
   can have its Unicode General_Category value change from So to Sm, or
   from Lo to Ll, without affecting the algorithm results.  Moreover,
   even if such changes were to result, the BackwardCompatible list
   (Section 9.7) can be adjusted to ensure the stability of the results.

9.  Category Definitions Used to Calculate Derived Property

   The derived property obtains its value based on a two-step procedure:

   1.  Code points are placed in one or more character categories either
       (1) based on core properties defined by the Unicode Standard or
       (2) by treating the code point as an exception and addressing the
       code point based on its code point value.  These categories are
       not mutually exclusive.

   2.  Set operations are used with these categories to determine the
       values for a property specific to a given string class.  These
       operations are specified under Section 8.

      Note: Unicode property names and property value names might have
      short abbreviations, such as "gc" for the General_Category
      property and "Ll" for the Lowercase_Letter property value of the
      gc property.

   In the following specification of character categories, the operation
   that returns the value of a particular Unicode code point property
   for a code point is designated by using the formal name of that
   property (from the Unicode PropertyAliases.txt file [PropertyAliases]
   followed by "(cp)" for "code point".  For example, the value of the
   General_Category property for a code point is indicated by

   The first ten categories (A-J) shown below were previously defined
   for IDNA2008 and are referenced from [RFC5892] to ease the
   understanding of how PRECIS handles various code points.  Some of
   these categories are reused in PRECIS, and some of them are not;
Top   ToC   RFC8264 - Page 25
   however, the lettering of categories is retained to prevent overlap
   and to ease implementation of both IDNA2008 and PRECIS in a single
   software application.  The next eight categories (K-R) are specific
   to PRECIS.

9.1.  LetterDigits (A)

   This category is defined in Section 2.1 of [RFC5892] and is included
   by reference for use in PRECIS.

9.2.  Unstable (B)

   This category is defined in Section 2.2 of [RFC5892].  However, it is
   not used in PRECIS.

9.3.  IgnorableProperties (C)

   This category is defined in Section 2.3 of [RFC5892].  However, it is
   not used in PRECIS.

   Note: See the PrecisIgnorableProperties ("M") category below for a
   more inclusive category used in PRECIS identifiers.

9.4.  IgnorableBlocks (D)

   This category is defined in Section 2.4 of [RFC5892].  However, it is
   not used in PRECIS.

9.5.  LDH (E)

   This category is defined in Section 2.5 of [RFC5892].  However, it is
   not used in PRECIS.

   Note: See the ASCII7 ("K") category below for a more inclusive
   category used in PRECIS identifiers.

9.6.  Exceptions (F)

   This category is defined in Section 2.6 of [RFC5892] and is included
   by reference for use in PRECIS.

9.7.  BackwardCompatible (G)

   This category is defined in Section 2.7 of [RFC5892] and is included
   by reference for use in PRECIS.

   Note: Management of this category is handled via the processes
   specified in [RFC5892].  At the time of this writing (and also at the
Top   ToC   RFC8264 - Page 26
   time that RFC 5892 was published), this category consisted of the
   empty set; however, that is subject to change as described in
   RFC 5892.

9.8.  JoinControl (H)

   This category is defined in Section 2.8 of [RFC5892] and is included
   by reference for use in PRECIS.

   Note: In particular, the code points ZERO WIDTH JOINER (U+200D) and
   ZERO WIDTH NON-JOINER (U+200C) are necessary to produce certain
   combinations of characters in certain scripts (e.g., Arabic, Persian,
   and Indic scripts), but if used in other contexts, they can have
   consequences that violate the "Principle of Least Astonishment".
   Therefore, these code points are allowed only in contexts where they
   are appropriate, specifically where the relevant rule (CONTEXTJ or
   CONTEXTO) has been defined.  See [RFC5892] and [RFC5894] for further

9.9.  OldHangulJamo (I)

   This category is defined in Section 2.9 of [RFC5892] and is included
   by reference for use in PRECIS.

   Note: Exclusion of these code points results in disallowing certain
   archaic Korean syllables and in restricting supported Korean
   syllables to preformed, modern Hangul characters.

9.10.  Unassigned (J)

   This category is defined in Section 2.10 of [RFC5892] and is included
   by reference for use in PRECIS.

9.11.  ASCII7 (K)

   This PRECIS-specific category consists of all printable, non-space
   code points from the 7-bit ASCII range.  By applying this category,
   the algorithm specified under Section 8 exempts these code points
   from other rules that might be applied during PRECIS processing, on
   the assumption that these code points are in such wide use that
   disallowing them would be counterproductive.

   K: cp is in {0021..007E}
Top   ToC   RFC8264 - Page 27
9.12.  Controls (L)

   This PRECIS-specific category consists of all control code points,
   such as LINE FEED (U+000A).

   L: Control(cp) = True

9.13.  PrecisIgnorableProperties (M)

   This PRECIS-specific category is used to group code points that are
   discouraged from use in PRECIS string classes.

   M: Default_Ignorable_Code_Point(cp) = True or
      Noncharacter_Code_Point(cp) = True

   The definition for Default_Ignorable_Code_Point can be found in the
   DerivedCoreProperties.txt file [DerivedCoreProperties].

   Note: In general, these code points are constructs such as so-called
   "soft hyphens", certain joining code points, various specialized code
   points for use within Unicode itself (e.g., language tags and
   variation selectors), and so on.  Disallowing these code points in
   PRECIS reduces the potential for unexpected results in the use of
   internationalized strings.

9.14.  Spaces (N)

   This PRECIS-specific category is used to group code points that are

   N: General_Category(cp) is in {Zs}

9.15.  Symbols (O)

   This PRECIS-specific category is used to group code points that are

   O: General_Category(cp) is in {Sm, Sc, Sk, So}

9.16.  Punctuation (P)

   This PRECIS-specific category is used to group code points that are

   P: General_Category(cp) is in {Pc, Pd, Ps, Pe, Pi, Pf, Po}
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9.17.  HasCompat (Q)

   This PRECIS-specific category is used to group any code point that is
   decomposed and recomposed into something other than itself under
   Unicode Normalization Form KC.

   Q: toNFKC(cp) != cp

   Typically, this category is true of code points that are
   "compatibility decomposable characters" as defined in the Unicode

   The toNFKC() operation returns the code point in Normalization
   Form KC.  For more information, see Unicode Standard Annex #15

9.18.  OtherLetterDigits (R)

   This PRECIS-specific category is used to group code points that are
   letters and digits other than the "traditional" letters and digits
   grouped under the LetterDigits ("A") category (see Section 9.1).

   R: General_Category(cp) is in {Lt, Nl, No, Me}

10.  Guidelines for Designated Experts

   Experience with internationalization in application protocols has
   shown that protocol designers and application developers usually do
   not understand the subtleties and trade-offs involved with
   internationalization and that they need considerable guidance in
   making reasonable decisions with regard to the options before them.


   o  Protocol designers are strongly encouraged to question the
      assumption that they need to define new profiles, because existing
      profiles are designed for wide reuse (see Section 5 for further

   o  Those who persist in defining new profiles are strongly encouraged
      to clearly explain a strong justification for doing so and to
      publish a stable specification that provides all of the
      information described under Section 11.3.

   o  The designated experts for profile registration requests ought to
      seek answers to all of the questions provided under Section 11.3
      and ought to encourage applicants to provide a stable
      specification documenting the profile (even though the
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      registration policy for PRECIS profiles is "Expert Review" and a
      stable specification is not strictly required).

   o  Developers of applications that use PRECIS are strongly encouraged
      to apply the guidelines provided under Section 6 and to seek out
      the advice of the designated experts or other knowledgeable
      individuals in doing so.

   o  All parties are strongly encouraged to help prevent the
      multiplication of profiles beyond necessity, as described under
      Section 5.1, and to use PRECIS in ways that will minimize user
      confusion and insecure application behavior.

   Internationalization can be difficult and contentious; designated
   experts, profile registrants, and application developers are strongly
   encouraged to work together in a spirit of good faith and mutual
   understanding to achieve rough consensus on profile registration
   requests and the use of PRECIS in particular applications.  They are
   also encouraged to bring additional expertise into the discussion if
   that would be helpful in adding perspective or otherwise resolving

(page 29 continued on part 3)

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